A method for classifying is known in which an auxiliary agent composed of a fluid such as an alcohol is added beforehand when classifying a powder, such as glassy blast furnace slag, into fine powder and rough powder (for example, see Patent Literature 1). In this method for classifying, the formation of aggregated particles with a large particle diameter due to adsorption and clumping together of particles is prevented by electrically neutralizing the polarity of the powder particles through the addition of an auxiliary agent containing polar molecules to the powder, thereby preventing a decline in the efficiency of classification.